When reading the book of Analysis of Financial Time Series, I have a question on the ARCH model, defined as follows

Regarding this model, the author also states that.

I do not understand why does the equation marked with yellow color is satisfied.
When reading the book of Analysis of Financial Time Series, I have a question on the ARCH model, defined as follows

Regarding this model, the author also states that.

I do not understand why does the equation marked with yellow color is satisfied.
Copyright © 2021 JogjaFile Inc.
$E(a_t^4|F_{t-1}) = E(\sigma_t^4\epsilon_t^4|F_{t-1}) = E(\epsilon_t^4|F_{t-1}) * E[(\sigma_t^2)^2|F_{t-1}] = 3(\alpha_0 + \alpha_1a_{t-1}^2)^2 $
I suggest reading up on the law of iterated expectations.
Ninja edit to your specific question is we know $E(\epsilon_t^4|F_{t-1})$ = 3